Bio-inspired, sub-wavelength surface structures for ultra-broadband, omni-directional anti-reflection in the mid and far IR

Abstract

Quasi-ordered moth-eye arrays were fabricated in Si using a colloidal lithography method to achieve highly efficient, omni-directional transmission of mid and far infrared (IR) radiation. The effect of structure height and aspect ratio on transmittance and scattering was explored experimentally and modeled quantitatively using effective medium theory. The highest aspect ratio structures (AR = 9.4) achieved peak transmittance of 98%, with >85% transmission for λ = 7-30 μm. A detailed photon balance was constructed by measuring transmission, forward scattering, specular reflection and diffuse reflection to quantify optical losses due to near-field effects. In addition, angle-dependent transmission measurements showed that moth-eye structures provide superior anti-reflective properties compared to unstructured interfaces over a wide angular range (0-60° incidence). The colloidal lithography method presented here is scalable and substrate-independent, providing a general approach to realize moth-eye structures and anti-reflection in many IR-compatible material systems.

Bio-Inspired Functional Surfaces Based on Laser-Induced Periodic Surface Structures

“…( b ) Adapted from Gonzalez and Gordon [77], Copyright 2014, with permission from OSA Publishing; ( d , e ) Adapted by permission from Macmillan Publishers Ltd.: Nature Communications, Siddique et al . [76], Copyright 2015.…”

Narrow-angle scatter of reflectivity-suppressing nanostructured surfaces

“…Recent studies have demonstrated that random antireflective structured surfaces (rARSS) can be used as an alternative to thin film coatings, reducing Fresnel reflections in the visible and the infrared (IR). [1][2][3][4][5][6][7][8] Broadband response, high transmission across wide ranges of angle of incidence (AOI), and polarization insensitivity have also been reported. 9,10 In general, the AR effects correlate with the density and depth of the nanostructures, simulating an effective medium gradient-index boundary profile, which reduces Fresnel reflectivity.…”

Manufacturing of highly ordered porous anodic alumina with conical pore shape and tunable interpore distance in the range of 550 nm to 650 nm

“…Moreover, the EMT predicts that better light entrapment will occur for larger pillar heights as long as p λ/ns. It was shown previously that transmission at long wavelengths is governed by the pillar height [17,18]. The height of the surface structures should be also sufficient to ensure a slowly varying volume fraction change.…”

“…The alumina taper-nanopores were used for preparing polymer AR coatings operating in visible and NIR range via template imprinting technique [18,25,26]. For an AR coating working at longer wavelengths, however, a larger pore spacing is needed [3,4,17]. Larger pore spacing means also the possibility to design tapered pores with a larger area of the cone base.…”